1. Field of the Invention
This invention relates to a system for detecting an abnormal shadow in a radiation image, and more particularly to a system for detecting a micro calcification shadow on the basis of image data representing a radiation image of an object.
2. Description of the Related Art
In the medical field, to find a diseased part of a patient or to observe a diseased part of a patient and diagnose progress of disease by reading a radiation image of the object (patient) has been a common operation. However, radiation image reading often depends upon experience and abilities of the reader and is not necessarily objective.
For example, it is necessary to find an abnormal shadow representing a growth and/or a micro calcification representative of a cancerous part in a mammogram (a radiation image of a breast) taken for the purpose of a breast cancer examination. However, depending on the reader, the abnormal shadow range cannot be properly detected. Accordingly, there has been a demand to properly detect an abnormal shadow including shadows of a growth and a micro calcification.
In order to meet this demand, there have been proposed abnormal shadow detecting systems (computer-aided image diagnosis systems), for instance, in Japanese Unexamined Patent Publication Nos. 8(1996)-294479 and 8(1996)-287230, in which an abnormal shadow is automatically detected by the use of a computer on the basis of image data representing a radiation image of the object. In the abnormal shadow detecting system, an abnormal shadow is automatically detected by the use of an iris filter processing which is mainly suitable for detecting a growth shadow and/or a morphology filter which is mainly suitable for detecting a micro calcification shadow.
The morphology filter processing will be briefly described hereinbelow. A morphology filter is a filter for removing or extracting from an image noises and/or shadows smaller than a structural element of a predetermined size, and is employed to smoothen image signals or to extract a micro calcification shadow representative of a cancerous part. The morphology filter processing is a method for detecting a micro calcification shadow representative of breast cancer through comparison of the output value of a morphology operation using a structural element larger than a micro calcification shadow (a discrete micro calcification shadow like a small point) to be detected with a predetermined threshold value.
(The Fundamentals of Morphology Operation)
Though the morphology operation is generally developed as a set theory in a N-dimensional space, it will be discussed here on the basis of a two-dimensional tone image for the purpose of simplicity of understanding.
It is assumed that a tone image is a space in which a point (x, y) has a height corresponding to a value of density f(x, y). Further it is assumed that the value of density f(x, y) is represented by a high brightness, high level signal in which as the value of density decreases (the value of brightness increases when displayed on a CRT), the level of the signal becomes higher.
For the purpose of simplicity, a linear function f(x) corresponding to a cross-section of the image is first discussed. It is assumed that a structural element g employed in the morphology operation is a function which is represented by the following formula (1), is symmetrical about the origin, and is 0 in value in a domain represented by the following formula (2).gs(x)=g(−x)  (1)G={−m, −m+1, . . . , −1, 0, 1, . . . , m−1, m }  (2)At this time, the fundamental form of the morphology operation is very simple as shown in the following formulae (3) to (6).dilation: [f⊕Gs](i)=max {f(i−m), . . . , f(i), . . . , f(i+m) }  (3)erosion: [f⊕Gs](i)=min {f(i−m), . . . , f(i), . . . , f(i+m) }  (4)opening: fg=(f⊖gs)⊕g  (5)closing: fg=(f⊕gs)⊖g  (6)
That is, the dilation processing is processing for searching a maximum value in the area whose width is ±m (a value determined according to the structural element B) and whose center is at the pixel of current interest, see FIG. 7A, and the erosion processing is processing for searching a minimum value in the same area, see FIG. 7B. The opening processing corresponds to searching a maximum value after searching a minimum value, and the closing processing corresponds to searching a minimum value after searching a maximum value. In other words, the opening processing smoothens the density curve f(x) from the low brightness side by removing protrusions in density (the portions which are higher in brightness than the surroundings) which are narrower than the mask size 2 m (see FIG. 7C), whereas the closing processing smoothens the density curve f(x) from the high brightness side by removing recesses in density (the portions which are lower in brightness than the surroundings) which are narrower than the mask size 2 m (see FIG. 7D).
In the case of a high density, high level signal in which as the value of density increases, the level of the signal becomes higher, the value of image signal for the value of density f(x) is reverse to that of a high brightness, high level signal. Accordingly, the dilation processing for a high density, high level signal corresponds to the erosion processing (FIG. 7B) for a high brightness, high level signal. Similarly, the erosion processing for a high density, high level signal corresponds to the dilation processing (FIG. 7A) for a high brightness, high level signal, the opening processing for a high density, high level signal corresponds to the closing processing (FIG. 7D) for a high brightness, high level signal, and the closing processing for a high density, high level signal corresponds to the opening processing for a high brightness, high level signal (FIG. 7C). Description will be made only on the high brightness, high level signal here unless otherwise mentioned.
(Application to Detection of a Micro Calcification Shadow)
As a method of detecting a calcification shadow, a subtraction method in which a smoothened image is subtracted from an original image is conceivable. Since it is difficult to distinguish a calcification shadow from an elongated non-calcification shadow (e.g., of a mammary gland, a blood vessel, and a mammary gland supporting tissue) by a simple smoothening method, there has been proposed morphology operation processing based on opening operation using multiple structural elements as represented by the following formula (7). See “Extraction of Microcalcifications on Mammogram Using Morphological Filter with Multiple Structuring Elements” (Journal of Academy of Electronics/Information/Communication D-II, vol. J75-D-II No. 7, pp. 1170 to 1176, July 1992) and “Basic Theory of Mathematical Morphology and its Application to Mammograms Processing” (MEDICAL IMAGING TECHNOLOGY, Vol. 12, No.1 January 1994)
                                                                        P                =                                  f                  -                                                            max                                              i                        ∈                                                  (                                                      1                            ,                            …                            ⁢                                                                                                                  ,                            M                                                    )                                                                                      ⁢                                          {                                                                        (                                                      f                            ⊖                                                          B                              ⁢                                                                                                                          ⁢                              i                                                                                )                                                ⊕                                                  B                          ⁢                                                                                                          ⁢                          i                                                                    }                                                                                                                                              =                                  f                  -                                                            max                                              i                        ∈                                                  (                                                      1                            ,                            …                            ⁢                                                                                                                  ,                            M                                                    )                                                                                      ⁢                                          {                                              f                                                  B                          ⁢                                                                                                          ⁢                          i                                                                    }                                                                                                                                       (        7        )            wherein Bi (i stands for 1, 2, 3 and 4) are four linear structural elements Bi shown in FIG. 8. When the structural elements Bi are larger than the micro calcification shadow to be detected, calcification shadows which are signal protrusions narrower than the structural elements Bi (a part of image the image signal of which fluctuates in a range spatially narrower than the structural elements B) are removed by opening processing. On the other hand, an elongated non-calcification shadow is left there as it is after the opening processing (calculation of the second term in formula 14) so long as it is longer than the structural elements Bi and its inclination (the direction in which the shadow extends) conforms to any one of the four structural elements Bi. Accordingly, by subtracting the smoothened image (the image removed with the calcification shadow) obtained by the opening processing from the original image f, a micro-structural image P containing therein only a small calcification shadow is obtained. This the concept of formula (7).
In the case of a high density, high level signal, closing processing is applied according to the following formula (8) in place of opening processing.
                                                                        P                =                                  f                  -                                                            min                                              i                        ∈                                                  (                                                      1                            ,                            …                            ⁢                                                                                                                  ,                            M                                                    )                                                                                      ⁢                                          {                                                                        (                                                      f                            ⊕                                                          B                              ⁢                                                                                                                          ⁢                              i                                                                                )                                                ⊖                                                  B                          ⁢                                                                                                          ⁢                          i                                                                    }                                                                                                                                              =                                  f                  -                                                            min                                              i                        ∈                                                  (                                                      1                            ,                            …                            ⁢                                                                                                                  ,                            M                                                    )                                                                                      ⁢                                          {                                              f                                                  B                          ⁢                                                                                                          ⁢                          i                                                                    }                                                                                                                                       (        8        )            
However, a non-calcification shadow equivalent to a calcification shadow in size can partly remain. In such a case, non-calcification shadows contained in the micro-structural image P obtained from formula (7) are further removed by the use of differential information based on a morphology operation according to the following formula (9).Mgrad=(½)×{f⊕λB−f⊖λB }  (9)As the value of Mgrad increases, the probability that the shadow is of a calcification increases. Accordingly, a prospective calcification image Cs (an image formed by extracted prospective points of micro calcification) can be obtained according to the following formula (10).If P(i, j)≧T1, and Mgrad(i, j)≧T2Then, Cs(i, j)=P else Cs(i, j)=0  (10)T1 and T2 are empirically determined threshold values.
Since a non-calcification shadow different from a calcification shadow in size can be removed only by comparison of the micro-structural image P obtained according to formula (7) and the threshold value T1, only the condition of the first term of formula (10), P(i, j)>T1 has to be satisfied in the case where there is no possibility that a non-calcification shadow equivalent to a calcification shadow in size remains.
Finally, the cluster area Cc of the calcification shadow is detected by a combination of a multi-scale opening operation and closing operation represented by the following formula (11).Cc=Cs⊕λ1B⊖λ3B⊕λ2B  (11)λ1 and λ2 are respectively determined by the maximum distance between calcification shadows to be fused and the maximum radius of an isolated shadow to be removed, and λ3=λ1+λ2.
In the case of high density, high level signal, the relation between the opening operation and the closing operation in formulae (9) and (10) is reversed.
In the morphology filter processing, whether each extracted point is correctly of a micro calcification shadow or a point extracted wrong is determined on the basis of density. That is, an extracted point which is low in density (an isolated shadow) is regarded as noise or the like and omitted, whereas an area solely consisting of high-density extracted points is detected as a cluster area of the micro calcification shadow.
However, there still remain a possibility that noise or the like is included in high-density extracted points and that there is a non-calcification shadow in detected cluster areas.
Further, a low-density micro calcification shadow can be included in an isolated shadow. Such a low-density micro calcification shadow will be referred to as “isolated micro calcification shadow”, herein below. In the conventional system where the extracted point is correctly of a micro calcification is determined on the basis of the density, it has been impossible to determine whether an isolated shadow is of noise or the like, or isolated micro calcification. Accordingly, it has been impossible to extract only isolated micro calcification shadows.
Further, conventionally, whether a micro calcification shadow really exists in a prospective area in an image of a breast detected by the morphology filter processing is determined on the basis of whether the number of the prospective points included in the prospective area is not smaller than 10% of the total number of the prospective points included in the overall image of the breast.
That is, since in a micro calcification shadow, fine calcic blocks irregular in shape are densely distributed, cluster areas in which the number of prospective points included therein is smaller than a predetermined threshold value (in proportion) are omitted.
Further, it has been known that micro calcification shadows in an image are irregular in size and density whereas noises are relatively regular in size and density.